The present invention relates to an ion irradiation system, of which beam line includes a leader and a trailer. A value of the beam current between the leader and the trailer is measured by a beam non-breaking method.
Ion irradiation systems, having a leader and a trailer at their beam lines, include an ion implantation system or an electron-beam exposure system that implants ions into a target or irradiates a target with ions. Those systems are widely used in semiconductor manufacturing. The ion implantation system is used in an ion implanting process, where impurity is doped into a semiconductor wafer. In this process, an amount of ions to be implanted needs to be accurately controlled. In general, a beam current of the ion beam is measured with a Faraday cup disposed behind or on both sides of the wafer, then the implanted ion amount is controlled by a dose-controller. Precise control of the ion amount to be implanted needs a correct measurement of an amount of beam current.
A measurement of beam current in the middle of a beam line requires a Faraday cup prepared in the middle of the beam line. This Faraday cup is disposed at a place where the cup does not touch the ion beam while the water is irradiated with ions, and moved to a measuring place where the cup traps the ion beam as needed. However, the cup cuts off the ion beam during the measurement, so that the wafer is not irradiated with the beam.
Such an ion implantation system cannot irradiate the wafer with ion beam simultaneously in measuring a beam current value. Various methods have been disclosed to overcome this problem. For instance, plural substrates are placed on a rotary disc prepared in a process chamber of a high-current ion implantation system. The disc is rotated while it is in pendulum movement, so that the surfaces of the substrates are scanned and irradiated with the ion beam. As a result, the ion is implanted into the substrates.
There is a technique that perforation, e.g., is provided to the disc, and ion beam running through the perforation is received with the Faraday cup for measuring. Another technique available in a medium-current ion implantation system is that Faraday cups are prepared on both sides of a substrate, and a scan of the substrate surface with ion beam is over-scanned beyond the Faraday cups, so that a current value of the ion beam is measured. Those techniques allow the high-current ion implantation system to measure the current value of ion beam at intervals of approx. 200 msec and the medium-current ion implantation system to measure the current value at a shorter intervals than that of the high-current system.
A conventional ion implantation system, which measures a beam current with a Faraday cup, must be quipped with the Faraday cup disposed close to a semiconductor wafer or close to a beam track for measuring a beam current at the foregoing short intervals while the wafer is irradiated with an ion beam. In other words, the Faraday cup breaks the ion beam during the measuring because of its operating principles. Thus the ion-beam track must be away at a given distance from the wafer during the measuring. On the other hand, the ion-beam track or the wafer must be moved while the wafer is irradiated with the ion-beam so that the ion-beam can arrive at the wafer. It is necessary to increase the moving speed and/or shorten the moving distance in order to shorten a time difference between a moment of measurement and a moment of irradiation. The foregoing high-current ion implantation system is an example of moving the wafer.
An example of moving the ion-beam track is available in the medium-current ion implantation system discussed previously. A substantial change of an ion-beam track is not practical because it requires large electrical field or magnetic field and needs a large space. In any case, the moving distance must be shortened, so that the Faraday cup needs to be placed close to the semiconductor wafer. In this case, the ion beam having passed nearby the wafer is to be measured.
However, the ion beam having passed nearby the wafer loses parts of its electric charges due to outgas, which is generated during the implantation from the resist applied to the wafer. A major component of the outgas is hydrogen gas. The ion collides with the outgas, so that the ion is neutralized into an atom. This collision reduces little kinetic energy, and the atom is doped as impurity into the substrate. A ratio of the ions neutralized depends partially on a pressure of the outgas; however, assume that 100 pieces of ions are accelerated. Then approx. 90 pieces out of 100 arrive at the substrate as they are as ions, and approx. 10 pieces collide with the outgas and lose their electric charges, thereby being neutralized. An amount of the impurities in the wafer is actually 100 pieces; however, the Faraday cup measures that approx. 90 pieces of impurities are doped. This problem is unavoidable because the ion-beam having passed nearby the wafer is measured.
A correcting method to overcome this problem is disclosed. This method utilizes the fact that the ratio of an out gas pressure vs. ions to be neutralized stays almost constant. A relational expression about the ratio of a pressure inside the chamber vs. the ions to be neutralized is determined in advance by experiment. Then during the implantation, a pressure in the chamber is measured, thereby correcting the values measured by the Faraday cup. In fact this method incurs an error of several % even after the correction because pressure distribution in the chamber varies with time and relational expressions determined by the experiences include errors. A malfunction of the pressure gage causes abnormality in the correction, and produces a failure. This phenomenon often happens just before a periodic maintenance of the pressure gage.
FIG. 6 shows an example of a conventional high-current ion implantation system. Perforation is provided to disc 8, and ion beam 5 passing through the perforation is received by Faraday cup 6, thereby measuring a beam current. This method allows measuring a value of the beam current at intervals of 200 msec. In fact ion beam 5 having passed through outgas 4 is measured, so that a less value of the ion beam than the value actually implanted into wafer 7 is output. Therefore, a pressure of process chamber 11 is measured in advance by a pressure gage (not shown), and the beam-current value measured is corrected based on the ratio of the ions neutralized by the pressure and outgas 4 vs. all the ions. However, this relational expression includes an error because a relation of the pressures at between the place where the pressure gage is disposed and the place where ion beam 5 passed is not always constant. Those factors inevitably produce an error of several % in a measured value of the beam current.
FIG. 7 shows another example of a conventional high-current ion implantation system. Rotary disc 8 is moved so that ion beam 5 does not hit wafer 7, and when disc 8 arrives at the place where disc 8 does not interrupt beam 5, the beam current is measured by Faraday cup 6. In this case, outgas 4 does not influence the measurement; however, the beam current can be measured only at intervals of movement of disc 8. In other words, the beam current can be measured at intervals of 20-30 seconds, and a change of the beam current during the interval cannot be measured.
An ion implantation system is provided. A beam line in this system has a leader and a trailer, and at least one non-beam-breaking beam-intensity measuring instrument is disposed between the leader and the trailer.